Segmenting datasets - Revision history

CynthiaBluteau: /* Minimum fft-length */

2022-07-14T00:03:22Z

Minimum fft-length

← Older revision		Revision as of 00:03, 14 July 2022
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	[[#fftlength\|Fig. 1]] provides a guide to the fft-length required for resolving different subrange as a function of the speed past the sensor, and <math>\varepsilon</math>. For instance, an fft-length of 4 s would resolve one decade of the inertial subrange at speeds past the sensor of 0.5 m/s and <math>\varepsilon\sim10^{-7}</math> W/kg. Longer segments would be required for slower flows or lower <math>\varepsilon</math>. At <math>\varepsilon\approx10^{-9}</math> W/kg, one decade of the inertial subrange would be resolved with an fft-length longer than 10s provided the speed was faster than 0.5 m/s.		[[#fftlength\|Fig. 1]] provides a guide to the fft-length required for resolving different subrange as a function of the speed past the sensor, and <math>\varepsilon</math>. For instance, an fft-length of 4 s would resolve one decade of the inertial subrange at speeds past the sensor of 0.5 m/s and <math>\varepsilon\sim10^{-7}</math> W/kg. Longer segments would be required for slower flows or lower <math>\varepsilon</math>. At <math>\varepsilon\approx10^{-9}</math> W/kg, one decade of the inertial subrange would be resolved with an fft-length longer than 10s provided the speed was faster than 0.5 m/s.

	Because the inertial subrange may be contaminated at the highest wavenumbers by instrument noise, we suggest using longer segments than the minimum shown in [[#fftlength\|Fig. 1b]]. This strategy also enables having a larger number of spectral observations to fit over the inertial subrange given the spectral resolution ~~also depends on~~ the fft-length.		Because the inertial subrange may be contaminated at the highest wavenumbers by instrument noise, we suggest using longer segments than the minimum shown in [[#fftlength\|Fig. 1b]]. This strategy also enables having a larger number of spectral observations to [[Spectral fitting\|fit]] over the inertial subrange given the spectral resolution is equal to the inverse of the fft-length.

	===Choosing segment-length===		===Choosing segment-length===

CynthiaBluteau: /* Choosing segment-length */

2022-07-14T00:01:32Z

Choosing segment-length

← Older revision		Revision as of 00:01, 14 July 2022
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	===Choosing segment-length===		===Choosing segment-length===
	The final segment length may be larger than the fft-length depending if you use block-averaging for [[Compute the spectra#specavg\|computing the spectra]]. The maximum segment length should be shorter than the largest [[Time and length scales of turbulence\|turbulent time scales]]. To increase the statistical reliability of the spectral observations, which is absolutely necessary when applying any [[Velocity decontamination by cospectral methods\|co-spectral techniques]] for motion decontamination of spectra, we recommend segment length that is 5x the [[#fftlength\|fft-length]] unless band-averaging is employed during the [[Compute the spectra#specavg\| spectral averaging]]. More details about the minimum degrees of freedom are given in the [[[velocity decontamination by cospectral methods]] wikipage.		The final segment length may be larger than the fft-length depending if you use block-averaging for [[Compute the spectra#specavg\|computing the spectra]]. The maximum segment length should be shorter than the largest [[Time and length scales of turbulence\|turbulent time scales]]. To increase the statistical reliability of the spectral observations, which is absolutely necessary when applying any [[Velocity decontamination by cospectral methods\|co-spectral techniques]] for motion decontamination of spectra, we recommend segment length that is 5x the [[#fftlength\|fft-length]] unless band-averaging is employed during the [[Compute the spectra#specavg\| spectral averaging]]. More details about the minimum degrees of freedom are given in the [[velocity decontamination by cospectral methods]] wikipage.

	{{FontColor\|fg=white\|bg=red\|text= Are the peaks in the MAVS data vortex shedding from the rings. Check the motion sensors onboard?}}		{{FontColor\|fg=white\|bg=red\|text= Are the peaks in the MAVS data vortex shedding from the rings. Check the motion sensors onboard?}}

CynthiaBluteau: /* Overlapping segments */

2022-07-13T23:53:06Z

Overlapping segments

← Older revision		Revision as of 23:53, 13 July 2022
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	----		----
	Return to [[Preparing_quality-controlled_velocities]]		Return to [[Preparing_quality-controlled_velocities\|Preparing quality-controlled velocities]]

CynthiaBluteau: /* Choosing segment-length */

2022-07-11T15:44:18Z

Choosing segment-length

← Older revision		Revision as of 15:44, 11 July 2022
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	The final segment length may be larger than the fft-length depending if you use block-averaging for [[Compute the spectra#specavg\|computing the spectra]]. The maximum segment length should be shorter than the largest [[Time and length scales of turbulence\|turbulent time scales]]. To increase the statistical reliability of the spectral observations, which is absolutely necessary when applying any [[Velocity decontamination by cospectral methods\|co-spectral techniques]] for motion decontamination of spectra, we recommend segment length that is 5x the [[#fftlength\|fft-length]] unless band-averaging is employed during the [[Compute the spectra#specavg\| spectral averaging]]. More details about the minimum degrees of freedom are given in the [[[velocity decontamination by cospectral methods]] wikipage.		The final segment length may be larger than the fft-length depending if you use block-averaging for [[Compute the spectra#specavg\|computing the spectra]]. The maximum segment length should be shorter than the largest [[Time and length scales of turbulence\|turbulent time scales]]. To increase the statistical reliability of the spectral observations, which is absolutely necessary when applying any [[Velocity decontamination by cospectral methods\|co-spectral techniques]] for motion decontamination of spectra, we recommend segment length that is 5x the [[#fftlength\|fft-length]] unless band-averaging is employed during the [[Compute the spectra#specavg\| spectral averaging]]. More details about the minimum degrees of freedom are given in the [[[velocity decontamination by cospectral methods]] wikipage.


	{{FontColor\|fg=white\|bg=red\|text= Are the peaks in the MAVS data vortex shedding from the rings. Check the motion sensors onboard?}}		{{FontColor\|fg=white\|bg=red\|text= Are the peaks in the MAVS data vortex shedding from the rings. Check the motion sensors onboard?}}



	[[File:Segment_anisotropy.png\|left\|thumbnail\|350px\|Fig. 2: Example theoretical velocity spectra for different <math>\varepsilon</math> with the empirical limit <math>\hat{k}L_k\sim0.1</math> denoted by the diamonds (<math>\hat{k}</math> is in rad/m). The inertial subrange extends to smaller wavenumber <math>k</math> [cpm] as <math>\varepsilon</math> increases. The lowest frequency resolved by a spectra is the inverse of the fft-length used when computing the spectra. The colored lines are spectral observations from a dataset with <span id="fastepsi">fast speeds and large</span> <math>\varepsilon</math>. In this example, we used relatively short segments (128s) to estimate the spectra from fft-length of 32 s (2048 samples @ 64 Hz). The impact of [[Velocity inertial subrange model#anisotropy\|turbulence anisotropy]] is also visible through the flattening of the spectra around 1 cpm. The secondary x-axis show the corresponding frequencies for a range of mean speeds past the sensors]]		[[File:Segment_anisotropy.png\|left\|thumbnail\|350px\|Fig. 2: Example theoretical velocity spectra for different <math>\varepsilon</math> with the empirical limit <math>\hat{k}L_k\sim0.1</math> denoted by the diamonds (<math>\hat{k}</math> is in rad/m). The inertial subrange extends to smaller wavenumber <math>k</math> [cpm] as <math>\varepsilon</math> increases. The lowest frequency resolved by a spectra is the inverse of the fft-length used when computing the spectra. The colored lines are spectral observations from a dataset with <span id="fastepsi">fast speeds and large</span> <math>\varepsilon</math>. In this example, we used relatively short segments (128s) to estimate the spectra from fft-length of 32 s (2048 samples @ 64 Hz). The impact of [[Velocity inertial subrange model#anisotropy\|turbulence anisotropy]] is also visible through the flattening of the spectra around 1 cpm. The secondary x-axis show the corresponding frequencies for a range of mean speeds past the sensors]]

CynthiaBluteau at 15:33, 11 July 2022

2022-07-11T15:33:23Z

← Older revision		Revision as of 15:33, 11 July 2022
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	* [[Time and length scales of turbulence]]		* [[Time and length scales of turbulence]]
	* [[Stationarity]] of the segment and [[Taylor's Frozen Turbulence\| Taylor's frozen turbulence hypothesis]]		* [[Stationarity]] of the segment and [[Taylor's Frozen Turbulence\| Taylor's frozen turbulence hypothesis]]
	* Required statistical significance of the resulting spectra, important mainly if you use [[Velocity decontamination by cospectral methods\| cospectral or coherence-based] methods to remove motion-induced contamination from the spectra		* Required statistical significance of the resulting spectra, important mainly if you use [[Velocity decontamination by cospectral methods\| cospectral or coherence-based]] methods to remove motion-induced contamination from the spectra

	A good way to select the segment length is by inspecting the [[compute the spectra\|computed spectra]] estimated from a 512-s long segment and an [[#fftlength\|fft-length]] that is one-quarter of the segment length (128 s). Plotting these spectra against the theoretical spectral lines (see [[#specavg\|Fig 2 and 3]]), enables identifying whether 1 decade of an inertial subrange is resolved with at least 8 spectral samples in this subrange.		A good way to select the segment length is by inspecting the [[compute the spectra\|computed spectra]] estimated from a 512-s long segment and an [[#fftlength\|fft-length]] that is one-quarter of the segment length (128 s). Plotting these spectra against the theoretical spectral lines (see [[#specavg\|Fig 2 and 3]]), enables identifying whether 1 decade of an inertial subrange is resolved with at least 8 spectral samples in this subrange.
Line 29:		Line 29:

	===Choosing segment-length===		===Choosing segment-length===
	The final segment length may be larger than the fft-length depending if you use block-averaging for [[Compute the spectra#specavg\|computing the spectra]]. The maximum segment length should be shorter than the largest [[Time and length scales of turbulence\|turbulent time scales]]. To increase the statistical reliability of the spectral observations, which is absolutely necessary when applying any [[Velocity decontamination by cospectral methods\|co-spectral techniques]] for motion decontamination of spectra, we recommend segment length that is 4-5x the [[#fftlength\|fft-length]].		The final segment length may be larger than the fft-length depending if you use block-averaging for [[Compute the spectra#specavg\|computing the spectra]]. The maximum segment length should be shorter than the largest [[Time and length scales of turbulence\|turbulent time scales]]. To increase the statistical reliability of the spectral observations, which is absolutely necessary when applying any [[Velocity decontamination by cospectral methods\|co-spectral techniques]] for motion decontamination of spectra, we recommend segment length that is 5x the [[#fftlength\|fft-length]] unless band-averaging is employed during the [[Compute the spectra#specavg\| spectral averaging]]. More details about the minimum degrees of freedom are given in the [[[velocity decontamination by cospectral methods]] wikipage.

CynthiaBluteau at 15:29, 11 July 2022

2022-07-11T15:29:47Z

← Older revision		Revision as of 15:29, 11 July 2022
Line 6:		Line 6:
	* [[Time and length scales of turbulence]]		* [[Time and length scales of turbulence]]
	* [[Stationarity]] of the segment and [[Taylor's Frozen Turbulence\| Taylor's frozen turbulence hypothesis]]		* [[Stationarity]] of the segment and [[Taylor's Frozen Turbulence\| Taylor's frozen turbulence hypothesis]]
	* Required statistical significance of the resulting spectra ~~(only~~ important if you ~~need~~ to remove motion-induced contamination from the spectra)		* Required statistical significance of the resulting spectra, important mainly if you use [[Velocity decontamination by cospectral methods\| cospectral or coherence-based] methods to remove motion-induced contamination from the spectra

	A good way to select the segment length is by inspecting the [[compute the spectra\|computed spectra]] estimated from a 512-s long segment and an [[#fftlength\|fft-length]] that is one-quarter of the segment length (128 s). Plotting these spectra against the theoretical spectral lines (see [[#specavg\|Fig 2 and 3]]), enables identifying whether 1 decade of an inertial subrange is resolved with at least 8 spectral samples in this subrange.		A good way to select the segment length is by inspecting the [[compute the spectra\|computed spectra]] estimated from a 512-s long segment and an [[#fftlength\|fft-length]] that is one-quarter of the segment length (128 s). Plotting these spectra against the theoretical spectral lines (see [[#specavg\|Fig 2 and 3]]), enables identifying whether 1 decade of an inertial subrange is resolved with at least 8 spectral samples in this subrange.

CynthiaBluteau: /* Choosing segment-length */

2022-07-11T15:28:49Z

Choosing segment-length

← Older revision		Revision as of 15:28, 11 July 2022
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	===Choosing segment-length===		===Choosing segment-length===
	The final segment length may be larger than the fft-length depending if you use block~~- or band~~-averaging for [[Compute the spectra#specavg\|computing the spectra]]. The maximum segment length should be shorter than the largest [[Time and length scales of turbulence\|turbulent time scales]].		The final segment length may be larger than the fft-length depending if you use block-averaging for [[Compute the spectra#specavg\|computing the spectra]]. The maximum segment length should be shorter than the largest [[Time and length scales of turbulence\|turbulent time scales]]. To increase the statistical reliability of the spectral observations, which is absolutely necessary when applying any [[Velocity decontamination by cospectral methods\|co-spectral techniques]] for motion decontamination of spectra, we recommend segment length that is 4-5x the [[#fftlength\|fft-length]].

CynthiaBluteau at 15:01, 11 July 2022

2022-07-11T15:01:56Z

← Older revision		Revision as of 15:01, 11 July 2022
Line 8:		Line 8:
	* Required statistical significance of the resulting spectra (only important if you need to remove motion-induced contamination from the spectra)		* Required statistical significance of the resulting spectra (only important if you need to remove motion-induced contamination from the spectra)

	A good way to ~~inspect~~ the [[compute the spectra\|computed spectra]] estimated from a 512-s long segment and an [[#fftlength\|fft-length]] that is one-quarter of the segment length (128 s). Plotting these spectra against the theoretical spectral lines (see [[#specavg\|Fig 2 and 3]]), enables identifying whether 1 decade of an inertial subrange is resolved with at least 8 spectral samples in this subrange.		A good way to select the segment length is by inspecting the [[compute the spectra\|computed spectra]] estimated from a 512-s long segment and an [[#fftlength\|fft-length]] that is one-quarter of the segment length (128 s). Plotting these spectra against the theoretical spectral lines (see [[#specavg\|Fig 2 and 3]]), enables identifying whether 1 decade of an inertial subrange is resolved with at least 8 spectral samples in this subrange.

	== Considerations ==		== Considerations ==

CynthiaBluteau at 15:01, 11 July 2022

2022-07-11T15:01:06Z

← Older revision		Revision as of 15:01, 11 July 2022
Line 7:		Line 7:
	* [[Stationarity]] of the segment and [[Taylor's Frozen Turbulence\| Taylor's frozen turbulence hypothesis]]		* [[Stationarity]] of the segment and [[Taylor's Frozen Turbulence\| Taylor's frozen turbulence hypothesis]]
	* Required statistical significance of the resulting spectra (only important if you need to remove motion-induced contamination from the spectra)		* Required statistical significance of the resulting spectra (only important if you need to remove motion-induced contamination from the spectra)

			A good way to inspect the [[compute the spectra\|computed spectra]] estimated from a 512-s long segment and an [[#fftlength\|fft-length]] that is one-quarter of the segment length (128 s). Plotting these spectra against the theoretical spectral lines (see [[#specavg\|Fig 2 and 3]]), enables identifying whether 1 decade of an inertial subrange is resolved with at least 8 spectral samples in this subrange.

	== Considerations ==		== Considerations ==

CynthiaBluteau: /* Choosing segment-length */

2022-07-11T14:52:06Z

Choosing segment-length

← Older revision		Revision as of 14:52, 11 July 2022
Line 27:		Line 27:

	===Choosing segment-length===		===Choosing segment-length===
	The final segment length may be larger than the fft-length depending if you use block- or band-averaging for [[Compute the spectra\|computing the spectra]] ~~{{FontColor\|fg=white\|bg=red\|text=Refer to textbook here?}}~~. The maximum segment length should be shorter than the largest [[Time and length scales of turbulence\|turbulent time scales]].		The final segment length may be larger than the fft-length depending if you use block- or band-averaging for [[Compute the spectra#specavg\|computing the spectra]]. The maximum segment length should be shorter than the largest [[Time and length scales of turbulence\|turbulent time scales]].


	{{FontColor\|fg=white\|bg=red\|text= Are the peaks in the MAVS data vortex shedding from the rings. Check the motion sensors onboard?}}		{{FontColor\|fg=white\|bg=red\|text= Are the peaks in the MAVS data vortex shedding from the rings. Check the motion sensors onboard?}}

← Older revision		Revision as of 15:44, 11 July 2022
Line 31:		Line 31:
	The final segment length may be larger than the fft-length depending if you use block-averaging for [[Compute the spectra#specavg\|computing the spectra]]. The maximum segment length should be shorter than the largest [[Time and length scales of turbulence\|turbulent time scales]]. To increase the statistical reliability of the spectral observations, which is absolutely necessary when applying any [[Velocity decontamination by cospectral methods\|co-spectral techniques]] for motion decontamination of spectra, we recommend segment length that is 5x the [[#fftlength\|fft-length]] unless band-averaging is employed during the [[Compute the spectra#specavg\| spectral averaging]]. More details about the minimum degrees of freedom are given in the [[[velocity decontamination by cospectral methods]] wikipage.		The final segment length may be larger than the fft-length depending if you use block-averaging for [[Compute the spectra#specavg\|computing the spectra]]. The maximum segment length should be shorter than the largest [[Time and length scales of turbulence\|turbulent time scales]]. To increase the statistical reliability of the spectral observations, which is absolutely necessary when applying any [[Velocity decontamination by cospectral methods\|co-spectral techniques]] for motion decontamination of spectra, we recommend segment length that is 5x the [[#fftlength\|fft-length]] unless band-averaging is employed during the [[Compute the spectra#specavg\| spectral averaging]]. More details about the minimum degrees of freedom are given in the [[[velocity decontamination by cospectral methods]] wikipage.


	{{FontColor\|fg=white\|bg=red\|text= Are the peaks in the MAVS data vortex shedding from the rings. Check the motion sensors onboard?}}		{{FontColor\|fg=white\|bg=red\|text= Are the peaks in the MAVS data vortex shedding from the rings. Check the motion sensors onboard?}}



	[[File:Segment_anisotropy.png\|left\|thumbnail\|350px\|Fig. 2: Example theoretical velocity spectra for different <math>\varepsilon</math> with the empirical limit <math>\hat{k}L_k\sim0.1</math> denoted by the diamonds (<math>\hat{k}</math> is in rad/m). The inertial subrange extends to smaller wavenumber <math>k</math> [cpm] as <math>\varepsilon</math> increases. The lowest frequency resolved by a spectra is the inverse of the fft-length used when computing the spectra. The colored lines are spectral observations from a dataset with <span id="fastepsi">fast speeds and large</span> <math>\varepsilon</math>. In this example, we used relatively short segments (128s) to estimate the spectra from fft-length of 32 s (2048 samples @ 64 Hz). The impact of [[Velocity inertial subrange model#anisotropy\|turbulence anisotropy]] is also visible through the flattening of the spectra around 1 cpm. The secondary x-axis show the corresponding frequencies for a range of mean speeds past the sensors]]		[[File:Segment_anisotropy.png\|left\|thumbnail\|350px\|Fig. 2: Example theoretical velocity spectra for different <math>\varepsilon</math> with the empirical limit <math>\hat{k}L_k\sim0.1</math> denoted by the diamonds (<math>\hat{k}</math> is in rad/m). The inertial subrange extends to smaller wavenumber <math>k</math> [cpm] as <math>\varepsilon</math> increases. The lowest frequency resolved by a spectra is the inverse of the fft-length used when computing the spectra. The colored lines are spectral observations from a dataset with <span id="fastepsi">fast speeds and large</span> <math>\varepsilon</math>. In this example, we used relatively short segments (128s) to estimate the spectra from fft-length of 32 s (2048 samples @ 64 Hz). The impact of [[Velocity inertial subrange model#anisotropy\|turbulence anisotropy]] is also visible through the flattening of the spectra around 1 cpm. The secondary x-axis show the corresponding frequencies for a range of mean speeds past the sensors]]